The phenomenon of superconductivity was first discovered in the early 1900s. A superconducting material conducts current with zero energy loss and expels magnetic field (like a perfect diamagnetic material) when cooled below its transition temperature. The transition temperature (Tc) of a superconducting material is called the critical temperature, and is a material-specific temperature. Until the mid-1980s all known superconducting materials were metallic compounds such as mercury (Hg), lead (Pb), and niobium-tin (Nb3Sn). In general, these materials become superconducting at temperatures below 35 degrees Kelvin, depending on the material, by undergoing a transition from the normal resistive state to the superconducting state. For any material in the superconducting state at a given temperature and applied magnetic field, there is a maximum current density that the material is able to conduct without developing resistance. This maximum current density is called the Critical Current Density (Jc). Jc for any superconductor is a function of both temperature (T) and external magnetic field (H). As the external magnetic field (H) or the temperature (T) increases, the critical current density Jc(T, H) decreases.
Depending on certain magnetization properties, a superconducting material can be characterized as a type I superconductor or a type II superconductor. When increasing the applied current or magnetic field, or raising the temperature above Tc, type I superconductors undergo a direct transition from the perfectly diamagnetic state (i.e., the Meissner state) to the normal state. Type II superconductors, however, first develop a “mixed (vortex) state,” wherein the applied magnetic field penetrates the superconducting material above the lower critical field (Hc1), and then the material undergoes the transition to the normal state above the upper critical field (Hc2). When the magnetic filed is raised above Hc1, it becomes energetically more favorable to admit into the material individual flux quanta (called fluxoids) in vortices than to maintain the Meissner state with the total flux exclusion. The vortices are distributed over the superconducting material to achieve an energetic minimum. When a transport current passes through the superconductor in the mixed state, the Lorentz force acts on the vortices, influencing the energy-minimizing pattern in which the vortices form. When the vortices move, electric field is generated which dissipates energy, and the material exhibits resistivity. Chemical and physical defects in the superconducting material can act to keep the vortices “pinned” at the location of the defect. Such defects are therefore referred to as pinning sites or pinning centers. Chemical and physical defects may be viewed as creating localized potential wells which act to hold fluxoids in place. The force that is necessary to break a fluxoid free of the attractive potential of a pinning center is called the pinning force. The pinning force of a given defect depends on the size, shape, orientation and composition of the defect, as well as on the distribution of surrounding defects.
As current flows in a superconducting material, if the Lorentz force (which is proportional to the current density) is less than the pinning forces, the vortices remain in place, and current density remains high. If the Lorentz force exceeds the pinning forces, the vortices start to move and dissipate heat, the material begins to exhibit resistivity, and the Critical Current Density, Jc, is reduced.
In the mid-1980s, the first high temperature superconductors (HTS) based on oxides of copper compounds were discovered. Some of these materials displayed superconductivity above liquid nitrogen temperature (Tc>77° K.), allowing dramatically more practical and economical cooling. For example, the HTS materials can be compounds of RE1Ba2Cu3O7−δ wherein RE abbreviates Y, or the Rare Earth elements, and the rare earth elements are Nd, La, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; compounds of Bi2Sr2CaCu2Ox, (Bi, Pb)2Sr2CaCu2Ox, Bi2Sr2Ca2Cu3Ox and (Bi, Pb)2Sr2Ca2Cu3Ox; compounds of TlCaBa2Cu2Ox or Tl2Ca2Ba2Cu3Ox; and compounds involving substitution such as the Nd1+xBa2−xCu3Ox compounds. These copper oxide superconductors are type II superconductors.
Several researchers have focused on introducing controlled defects into the HTS materials to increase the pinning forces. These defects reduce the movement of the fluxoids and permit high critical currents even at relatively high temperatures and high magnetic fields. Throughout the remainder of this document, unless explicitly stated otherwise, the term “defect” will be used to refer exclusively to intentionally created defects which act as pinning centers.
Magnetic field that penetrates the superconducting material may also lead to “trapped” magnetic field. The trapped field can be pinned in place even when there is no supporting external magnetic field. An ingot of superconducting material with trapped magnetic field not supported by an external magnet is called a trapped field magnet, and is similar in some ways to a permanent magnet. HTS materials with optimally placed and sized defects exhibit higher Jc, higher pinning forces, and can support higher trapped magnetic field than HTS materials without defects. HTS materials with pinning centers which are neither of optimum size, nor optimally placed, can improve Jc, the pinning forces, and the trapped field to a lesser extent.
When the externally applied magnetic field (or the applied current) is removed, the trapped magnetic fields decay over time. This decay over time is called flux creep. Flux creep tends to stabilize the flux distribution in the superconducting material by relieving the magnetic pressure. Flux creep, which decays approximately logarithmically over time, is a measure of loss of the trapped magnetic field. In a trapped field magnet, because the HTS has near zero resistivity, the current is persistent except for the slow decrease described by creep. The magnetic flux density is held in place by the pinning force and is related to the current density by Ampere's law. If the current density is increased toward the critical current density, Jc(T), flux creep increases.
It is usually desirable for the field in a trapped field magnet to stay stable over time. Optimal sizing and distribution of pinning sites allows for trapped-field magnets capable of supporting higher magnetic fields which are more stable over longer periods of time.
It is also desirable in a current-carrying superconducting wire or tape for the field to stay stable, so that the wire or tape does not develop resistivity, and dissipate energy.
As mentioned above, particular defects increase pinning of the fluxoids. The optimal size, shape, orientation, and distribution of defects depend on the superconductor used, on the direction of the field, and on the temperature. It depends primarily on a parameter (characteristic) of the superconductor called the coherence length, which determines the distance into the superconductor (from the defect) it takes the vortex current of the fluxoid to build up.
Murakami et al (Ref. 3) reported improved Jc by diminishing the size of second phase material in a high temperature superconductor. The superconductor YBa2Cu3O7, hereafter called Y123, for example, when formed by a process called melt texturing, has islands of Y2BaCuO5, hereafter called Y211, a non-superconductor, included as a second phase. This is an inclusion containing only elements normally found in the superconductor. We shall call these native elements. Murakami showed that when the Y211 islands are diminished in size, and/or elongated, they can act as a native chemical pinning centers. We refer to the process of diminishing size and/or elongation of a second phase of native elements by the term “refinement”.
Elements not normally found in superconductors may enhance refinement. We call elements not normally found in superconductors foreign elements. C. J. Kim, et al (Ref. 6) showed that Ce improved refinement of the Y211 second phase deposits in Y123.
Sawh et al (Ref. 4) reported improved Jc by forming deposits containing both native elements and foreign elements. Pt was added to the superconductor Y123 in order to refine the second phase deposits of Y211. When U was also added, upon texturing, deposits of a compound of U, Pt, Y, Ba and oxygen formed which acted as pinning centers. Weinstein (Ref. 5) showed that Jc was increased by chemical deposits containing uranium.
Texturing, as defined herein, includes any process of aligning microcrystals or growing larger crystals in a bulk sample, and also includes “natural” texturing that occurs when a thin film is deposited by any of the known physical deposition methods (for example, sputtering, evaporation, epitaxial growth) and is processed in-situ or ex-situ, or when a thick film is deposited (for example, by spin coating). Without texturing, polycrystalline HTS has very low intergrain current density.
In U.S. Pat. No. 6,083,885, Weinstein (Ref. 5) reports improved JC in a textured superconductor including a defect compound containing uranium and platinum. Weinstein also reports dramatically improved JC in a textured superconductor who's defect compound contains uranium and wherein the uranium is fissioned by neutron bombardment as part of the preparation method of the superconductor. Although the uranium-doped superconductors reported by Weinstein exhibit great performance characteristics, public fear of materials which are even mildly radioactive has all but prevented any commercial use of these materials.
In general, there is a need for a non-radioactive high-Tc superconducting material with Tc above 77° K. and high values of JC and Hirr (the maximum magnetic field in which the superconductor can conduct current as a superconductor), which can be economically produced in uniform bulk quantities, or in form of thick or thin films, and which is suitable for different superconducting applications. Refinement of second phase pinning centers, composed of native elements, or formation of deposits of compounds containing foreign elements, can be used to create pinning centers to accomplish these aims.